A.8 Failure Scenarios

This section lists a number of possible failure scenarios that might be expected for offshore carbon capture platforms and pipelines. It is not intended to be exhaustive, but it is intended to cover the whole offshore transport chain. In producing this, it is appreciated that not all failure scenarios are of equal likelihood; indeed, some are very unlikely in the North Sea. It is also appreciated that not all of the failures would lead to incidents of equal severity. Some comment on both the likelihood and severity of the scenario is included, but only at a relatively coarse level of small, medium and large.

The main hazard associated with a subsea pipeline is loss of containment resulting in a gas release, which could be initiated by a vessel in the vicinity (third party interference, see A.2.4.1). The principal immediate causes for loss of containment from a subsea pipeline are as follows:

- TPD due to anchor drop/drag, vessel sinking/grounding, objects dropped by passing vessels (e.g. construction tubulars and shipping containers).

- Corrosion: internal corrosion is not expected for the proposed pipelines; however, external corrosion could occur due to failure or breakdown of the corrosion protection system.

- Mechanical failure, including material defect, weld failure, seal leakage, etc.

- Construction related damage, e.g. from failure of equipment on the pipeline laying barge, dropped/dragged anchors, dropped objects, pipe buckle and soil breakdown leading to mis-positioning of the pipeline.

- Natural hazards, such as subsidence, earthquake and typhoon.

As for the offshore pipelines, failure in the subsea pipelines could manifest as a pinhole leak, hole or a rupture.

The offshore chain has been divided into two main sections, as follows:

A.8.2: Pipeline between the beachhead and the wellhead.

A.8.3: Platforms onto which CO2 has been brought.

A 'traffic light' annotation has been used (green, low; amber, medium; red, high) to highlight both the likelihood and severity of possible failure scenarios. Many of these have been described as having a historically low frequency rate (HLFR).

Table A.6 General observations on credible events